NSW Health is urging people to be on the lookout for the signs and symptoms of the disease, which you'll find listed below.

Perhaps most concerning when a measles outbreak occurs, such as the one which has broken out in NSW over the Christmas/New Year period, is the fact that the disease has an incubation period of between seven to 18 days, meaning symptoms of the highly contagious disease take a while to show up.

Those two cases follow an earlier case of a young adult from Sydney who was diagnosed with measles after returning from Thailand on December 29.

Measles is highly contagious and is spread in the air through coughing or sneezing.

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Who is at risk?

People most vulnerable to a measles infection are infants under 12 months, who are too young to be vaccinated, and young adults who may not have had two doses of the MMR vaccine.

The national immunisation register sees children immunised at 12 months of age with the measles mumps rubella (MMR) vaccine and again at 18 months with measles mumps rubella varicella (MMRV).

NSW health officials want people to know that two doses of the measles vaccine will provide lifelong protection against measles infection in 99 percent of the population. Those uncertain if they've had the second dose are able to safely receive another one.

People most vulnerable to a measles infection are infants under 12 months, who are too young to be vaccinated, and young adults who may not have had two doses of the MMR vaccine. Image: Getty.

Signs and symptoms of the measles

The most distinctive symptom of the measles is a rash which often starts on the face and spreads to the rest of the body. Other symptoms of the measles can resemble those of the common cold.

Fever

General discomfort, illness or lack of wellbeing (malaise)

Runny nose

Dry cough

Sore and red eyes (conjunctivitis)

Red and bluish spots inside the mouth (Koplik's spots)

Red and blotchy skin rash that appears first on the face and hairline, and then spreads to the body

If you, or someone you know, is showing symptoms, seek medical assistance immediately.